This invention relates to a liquid level sensing device useful in a fuel tank for a vehicle, for example, to sense the fuel level therein. The subject liquid level sensing device utilizes a conductive hollow sphere which floats on the liquid surface within a vertically oriented tube. Fuel is free to flow into and out of the tube so that the level of fuel within the tube is the same as the fuel level in the surrounding tank. An elongated resistance card is mounted in the tube in a manner so that the hollow metal sphere slides along the resistance card. Another portion of the hollow metal sphere touches the opposite wall of the metal tube thereby providing a variable resistance connection between the card and the ground which is connected to the metal tube.
A number of prior liquid sensing devices have utilized level sensors with a floating sphere. The U.S. Pat. No. 2,305,973 to Marchment utilizes a metal ball which is buoyed by the liquid level and slides against a wire wound resistance element in the form of an elongated rod with wraps of wire thereabout. The floating metal ball is biased against the wire wound resistor by means of an electromagnetically energized biasing device so that when the electromagnet is energized, the ball is pressed against the wire wound resistance so that a liquid level reading may be taken. It is not possible with this device to continuously monitor liquid levels since the energization of the electromagnet firmly presses the metal ball against the resistance coil and prevents the ball from sliding therealong when the liquid level changes. A similar structure is also shown in U.S. Pat. No. 2,260,913 to Marchment.
A number of patents utilize floats which are guided by stationary means as they move with liquid level changes. The following U.S. patents are of this type: U.S. Pat. Nos. 3,348,413 to Zimmerle; 3,433,073 to Kunert; 3,555,904 to Lenker; 3,813,941 to Miguel; 4,178,802 to Yamamoto. Some of the above patents utilize floats which travel along vertically oriented and elongated resistance devices. Other patents disclose the use of a plastic type resistance card device which is also used in the subject application. In this regard, U.S. Pat. No. 4,052,901 to Bjork discloses the use of a flexible plastic transducer for liquid level sensing. The flexible transducer is provided with a photo-etched pattern of electrical conductor. Similarly, U.S. Pat. No. 4,157,038 to Yamamoto utilizes a similar resistance card type transducer.